There's a kitchen stereotype that bakers are meticulous and measure everything to the gram. They follow directions to the letter, all while savory cooks are throwing food into pots Swedish Chef style.
Are Dutch Process and Natural Cocoa Interchangeable?
Natural Cocoa Powder
If you're making natural cocoa powder, that's the end of the line. Chocolate is naturally acidic, so natural cocoa powder typically has a pH between 5 and 6 (for context, water is 7, right in the middle). That acidity bears out in natural cocoa's flavor, which gives the cocoa a sharp, almost citrus fruit finish. Remember, that just like a chocolate bar, cocoa powder flavor varies by brand. While all natural cocoas will have certain characteristics in common (bitterness and astringency), flavors will vary based on the cacao bean and how it's manufactured. In most U.S supermarkets, natural cocoa is the most commonly available variety of cocoa—think Hershey's, Ghirardelli, and Scharffen Berger.
Dutch process cocoa has a smoother, more mellow flavor that's often associated with earthy, woodsy notes. There are also heavily Dutched "black" cocoa powders that bring the cocoa powder to an alkaline level of 8. This the kind of bittersweet cocoa you'll find in Oreo cookies.
Since Dutch process cocoa isn't acidic, it doesn't react with alkaline leaveners like baking soda to produce carbon dioxide. That's why recipes that use Dutch process cocoa are usually leavened by baking powder, which has a neutral pH.
I tested two chocolate cakes; the first called for baking powder, a mix of baking soda and acidic cream of tartar that, when hydrated and heated, leavens baked goods all on its own without the need of additional acids. In my test, both cakes did rise, but the cake with Dutch process cocoa was slightly darker and fudgier. The cake with natural cocoa powder produced a lighter final result with a slightly more open crumb structure.
(Note the lightly reddish hue of the cake using natural cocoa. This is actually one of the inspirations for red velvet cake, which was originally made with a devil's food cake recipe that highlighted the redness of natural cocoa powder).
The Dutch process cake was richer and had a deeper "chocolate" flavor, while the natural was more mild. If I had to choose a cake, I'd go for the one with Dutch process cocoa—that rich color, tighter crumb, and deep toasted vanilla flavor are hard to resist.
The chocolate pudding using the Dutch process cocoa was much darker in color, but no more or less creamy than the natural version. The Dutch process pudding had a richer flavor, while the natural pudding was lighter and a little more acidic. In recipes like this, your choice of cocoa is all about your personal taste: rich and dark or light and tangy.
With something liquid like hot chocolate, the major cocoa differences you'll see are color and flavor. The Dutch process version had an earthier but flatter flavor; the natural cocoa version was fruitier, but also more acidic. Again, the choice comes down to what you want your hot cocoa to taste like. In this situation, I recommend playing around with both types of cocoa powder as well as different brands and flavor pairings. For example, if you were making spicy hot chocolate with cayenne pepper and cinnamon, you might want the fruity astringency of natural cocoa, while a hazelnut hot chocolate would pair nicely with a more earthy Dutch processed cocoa.
What if the Recipe Doesn't Say?
Many recipes don't specify whether they call for natural or Dutch process cocoa, but American recipes tend to use natural, as that's what you'll find from most American supermarket brands. (Hershey's, for instance, is a natural cocoa.) When in doubt, stick to the leavening rule: recipes that rely on neutral-pH baking powder for leavening are best with similarly neutral pH Dutch process cocoa; those that are leavened by baking soda should stick to natural cocoa powder. If the recipe calls for both baking powder and baking soda, either will work, but it's best to stick to what the recipe calls for to get ideal results.